1. Field of the Invention
The invention relates to an optical disk apparatus and a method for adjusting optical disk apparatus laser power, and relates in particular to an optical disk apparatus and a method for adjusting optical disk apparatus laser power, permitting compensation for factors causing fluctuation in effective power.
2. Description of the Related Art
In recent years, with the digitalization of video data, audio data, and various other types of data, there has been a dramatic increase in digital data volume. Accompanying this, development of optical disk apparatuses suited for high-capacity, high-density applications has proceeded apace. Along with progress in achieving increased density in optical disk apparatuses there has been decreased signal quality in the readout signal, and there have been a great many inventions proposing various methods for obtaining satisfactory signal quality. The factors affecting the signal quality of the readout signal include not only the conditions under which readout takes place and the method employed for carrying out readout, but also the conditions under which recording takes place and the method employed for carrying out recording. Among such recording conditions, the laser power employed at the time of recording will in particular have a large effect on signal quality, and an invention concerned with optimization of the laser power employed at the time of recording is disclosed at Japanese Unexamined Patent Publication JP-A 11-73700 (1999).
JP-A 11-73700 discloses a procedure for determining an optimum laser power to be employed during recording wherein laser power is varied while carrying out recording at the active layer of the disk and the signal recorded at the disk active layer is thereafter read, with optimum laser power during recording being the laser power producing the greatest amplitude ratio in the readout signal. The sequence of recording operations employed to obtain this optimum laser power for use during recording is called a test write. By carrying out such a test write, it is possible to compensate for such factors as variation in laser power due to apparatus and disk characteristics as well as fluctuation in laser power due to changes in ambient temperature, employment of such an optimized laser power allowing an optical disk apparatus to record data with satisfactory signal quality.
In an optical disk apparatus it is possible for the laser power arriving at the disk active layer, i.e., the “effective power,” to fluctuate despite emission of constant laser power from the laser source, and this will tend to cause the optimum laser power to fluctuate as well. As a result, even for the same apparatus-disk combination and even where temperature is held constant, optimum laser power will in general in such a case be nonconstant due to varying conditions at the apparatus, resulting in the problem that it is difficult to cause the laser to irradiate the disk active layer with optimum laser power.
Included among the factors contributing to fluctuation in effective power are objective lens shift, tilt, and servo residual error, and the like. In particular, the effect of shift at the objective lens used in the optical head, i.e., displacement of the center of that objective lens relative to the optical axis of the laser, is large, such objective lens shift being capable of causing fluctuation in the optimum laser power during recording of as much as several percent.
The reason that objective lens shift causes fluctuation in effective power is that objective lens shift causes a change in the fraction of the laser light emitted from the laser source which is incident at the objective lens. Accordingly, despite use of APC (Automatic Power Control) to maintain a constant laser power emitted from the laser source, the laser power exiting the objective lens will nonetheless fluctuate, making it impossible to irradiate the disk active layer with constant effective power.
FIG. 6 is a graph showing laser power exiting an objective lens as a function of shift at the objective lens, the horizontal axis indicating the amount of shift at the objective lens and the vertical axis indicating the laser power exiting the objective lens. Holding laser power emitted from the laser source constant and varying the amount of shift at the objective lens, the laser power exiting the objective lens is a maximum for zero shift at the objective lens, i.e., when the center of the objective lens coincides with the optical axis of the laser. Furthermore, the laser power exiting the objective lens decreases with increasing shift at the objective lens; that is, as the objective lens is displaced in either a positive or a negative direction from the zero-shift arrangement.
Because optical head assembly tolerances cannot be completely eliminated, there will in general be a nonzero shift at the objective lens, i.e., the objective lens will be displaced in either a positive or a negative direction relative to the laser optical axis, when the objective lens is in its neutral state, i.e., at a time when it is not acted upon at all by the objective lens actuator. Moreover, during tracking servo operations, because the position of the objective lens is controlled so as to cause the beam spot to follow any eccentricity in the disk, the objective lens will furthermore be shifted in either a positive or a negative direction from its neutral position by an amount corresponding to such eccentricity. Another cause of objective lens shift is lag in tracking during seek servo operations for moving the optical head in the radial direction of the disk. For these and other reasons, occurrence of objective lens shift is unavoidable.
As described above, a change in objective lens shift will cause a change in the laser power exiting the objective lens, and the amount of objective lens shift at the time that the above-described test write is carried out may be different from the amount of objective lens shift at the time that a user carries out recording or readout. That is, there is an error component due to fluctuation in laser power caused by objective lens shift which is in general present despite the fact that the laser power exiting the laser source at the time of recording is an optimum laser power as determined by carrying out a test write.
Accordingly, as conventional optical disk apparatuses do not permit compensation for variation in optimum laser power due to changes in objective lens shift and other such factors causing fluctuation in effective power, there is the problem that recording is carried out at a laser power other than the optimum laser power at the time of recording, causing decrease in readout signal quality.